Young Shiveluch: 56°38' N, 161°19' E, elevation: active dome about 2800 m, summit of Old Shiveluch 3283 m; Last eruption 2008
Following facts about Shiveluch make it a unique volcano and attract attention of different specialists:
- it is considered to be the northernmost active volcano of Kamchatka
- it is noted for its unusual rocks, close to adakites, likely indicating its position over the northern edge of the subducting Pacific plate, warmed by mantle flow (Volynets et al. 2000; Yogodzinski et al. 2001)
- it is one of the most prolific explosive centers of Kamchatka, with a magma discharge of about 36x106 tons per year, an order of magnitude higher than that typical of island arc volcanoes (Melekestsev et al. 1991)
- it has experienced repetitive large flank failures, most recently in 1964 (Gorshkov and Dubik 1970; Belousov, 1995; Belousov et al., 1999; Ponomareva et al., 1998)
Shiveluch volcanic edifice has a complex structure: Late Pleistocene Old Shiveluch stratovolcano, complicated at its western flank with a later lava field (Baidarny ridge), is truncated by a 9-km-wide crater enclosing Young Shiveluch eruptive center, active through the Holocene. Several Holocene domes have been also emplaced at the western slopes of Old Shiveluch.
Written records of Shiveluch activity date back to 1739 and include two large plinian eruptions, in 1854 and 1964, and more than 10 moderate dome-associated events, which produced minor pyroclastic flows and ashfalls (Gorshkov, Dubik 1970; Dvigalo 1984; Gorelchik et al. 1995; Khubunaya et al. 1995; Zharinov et al. 1995). The last eruption of this kind in May 2001 caused 30-km long lahars (Fedotov et al. 2001). Due to its frequent and large explosive eruptions, Shiveluch poses hazard not only to towns of Kliuchi and Ust'-Kamchatsk, located at a distance of 45-85 km, but also for aviation pathways between the USA and all of the Far East.
Deep river cuts at the foot of the volcano expose a variety of its deposits. For example, a beautiful outcrop 13 km southeast of the volcano exposes two pyroclastic flow units and numerous fall and surge layers. Just few hundreds of meters up the valley we can see a debris avalanche unit, which is somewhat younger than the upper pyroclastic flow of Fig.5. Another few hundreds of meters up the valley - and above the debris avalanche deposit we see white, gray and black pyroclastic flow units, presumably related to the eruption immediately following the avalanche. In this way, tracing the pyroclastic units along the valleys and digging pits in places, where outcrops are lacking, we have mapped the deposits of the largest eruptions, which are often interlayered with organic-rich paleosols. In addition, many of pyroclastic flow units contain charcoals. Radiocarbon dating of this organic matter has allowed us to estimate the ages of Shiveluch tephras and reconstruct its eruptive history for the last 10 000 yrs. First results of these studies were published in Ponomareva et al. (1998, 2002) and Pevzner et al. (1998).
Well-known 1964 Shiveluch eruption included a large-scale slope failure, small phreatic explosion and a powerful plinian eruption resulting in pyroclastic fall and flows accompanied by lahars (Gorshkov, Dubik 1970; Belousov 1995). However, eruptive history of Shiveluch shows that the 1964 eruption, with its 1.5-2 km3 of debris avalanche deposit (Fig.10) and 0.6-0.8 km3 of pyroclastic products, was only a moderate event and had far larger predecessors. The largest Holocene eruptions of Shiveluch produced 1-3 km3 of tephra. Such eruptions occurred at least seven times during the last 10,000 years, most recently around 1650 (SH1) and 1000 AD (SH2). Eruptions with tephra volumes of 0.5-1 km3, close to the 1964 one, were more common and numbered at least 25 events. Smaller explosive eruptions with tephra volumes down to 0.01 km3 numbered in tens. Total number of Holocene eruptions, which have left a well identified tephra or debris avalanche unit at a distance of more than 8-10 km from the volcano, exceeds a hundred, so the minimum recurrence rate is one eruption per hundred years.
The last 2000 years are noted for continuous high activity and enclose 9 eruptions with tephra volumes of 0.5-2 km3 and four large (1-3 km3) and four smaller debris avalanches. One of the most recent large eruptions is SH1, which occurred only about 250 14C years ago (~1650 AD) and covered the southern foot of the volcano with a 20-km-long and 20-km-wide ignimbrite sheet. Tephra from this eruption, combining plinian fallout and co-ignimbrite ash, was deposited at a distance of more than 100 km from the source. Frequent large eruptions and high production rate set the last 2000 years apart from any other 2000-yrs-long period in the Shiveluch eruptive history. It is hardly probable that this can be explained by underrepresentation of the older deposits. More likely we are witnessing general intensification of Shiveluch activity.
Holocene eruptive products of Shiveluch are dominated by medium-K, high-Mg andesite. This monotonous composition was broken by two basaltic tephras: a unique high-K, high-Mg amphibole- and phlogopite-bearing basaltic tephra about 3600 14C yrs BP and medium-K high-Mg tephra about 7600 14C yrs BP (Volynets et al. 1997). Each of these eruptions fell into a period of especially high activity and was an evidence of some important events in the magmatic system under Shiveluch.
We, tephrochronologists, love Shiveluch dearly because its numerous ashfall layers can be traced over vast territories and thus serve as excellent markers in Holocene studies, including dating of other volcanic deposits and landforms, tsunami and landslide deposits, river and marine terraces, archaeological sites etc. (Braitseva et al., 1997). An extent of a compacted ash layer from a single eruption, preserved in the soil-pyroclastic cover, exceeds 400 km.
Major volcanic hazard at a distance of up to 30 km from Shiveluch is associated with lahars which were triggered both by eruptions and heavy rains. The river valleys on southern and northwestern slopes provide pathways for pyroclastic flows, some of which traveled over a distance of 30 km. Maximum runout of Holocene debris avalanches did not exceed 20 km. More distant localities are exposed to hazards associated with ash falls. In Kliuchi town, thickness of a layer of compacted ash from a single Shiveluch eruption exceeds 9 cm, and was likely two times over this value during deposition.